In extracting petroleum from underground reserves, a bore is drilled deep into the earth. Such bores are formed by connecting a drill bit to a long pipe, referred to as a "drill pipe," so as to form an assembly commonly referred to as a "drill string" that extends from the surface to the bottom of the bore. The drill string is rotated, thereby causing the drill bit to advance into the earth, forming the bore. In order to lubricate the drill bit and flush cuttings from its path, a fluid, referred to as "drilling mud," is directed through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through the annular passage formed between the drill string and the surface of the bore. Since the drilling mud must be highly pressurized, the drill string is subjected to a large pressure gradient in the radial direction, as well as high axial and torque loading due to the forces associated with rotating and advancing the drill bit and carrying the weight of the drill string; Consequently, the drill pipe must be especially strong. Moreover, since it is often necessary to form a curved bore, the drill pipe must also be flexible.
Traditionally, drill pipes have been formed by connecting sections of steel pipe, typically in lengths of about 30 feet. However, more recently, it has been proposed that drill pipes include sections of pipe formed from a composite material. According to one such approach, sections of composite pipe are interconnected using metallic couplings threaded on one end. The unthreaded end of the metallic coupling is bonded by an adhesive to an end of the composite pipe section, and the metallic couplings of adjacent composite pipe sections are threaded onto each other to form an assembly. Composite/metal pipe joints of this type are disclosed in U.S. Pat. No. 5,332,049 (Tew).
In addition to the drill bit, the distal end of a drill string, referred to as the "bottom hole assembly," often incorporates specialized sections, such as a stabilizer section, a sensing section, and an instrumentation/electrical section. These sections provide the drill operator with information concerning the formation being drilled through using techniques commonly referred to as "measurement while drilling" (MWD) or "logging while drilling" (LWD). In some cases, this information is used to control the direction in which the drill bit advances.
The sensor section may contain many different sensors some of which may include a transmitter and one or more receivers. The transmitter generates high frequency wavelength signals (e.g., electromagnetic waves) that travel through the formation surrounding the sensor and are then received by the receiver. By comparing the transmitted and received signals, information can be determined concerning the nature of the formation through which the signal traveled, such as whether it contains water or hydrocarbons. One such method for sensing and evaluating the characteristics of the formation is disclosed in U.S. Pat. No. 5,144,245 (Wisler), hereby incorporated by reference in its entirety. Other sensing methods under development include magnetic resonance imaging (MRI) such as that disclosed in U.S. Pat. No. 5,280,243 (Miller), hereby incorporated by reference in its entirety. Regardless of the method used, the information from the sensing section is typically transmitted to the surface so that the drilling personnel can use it in guiding the path of the drill bit.
The sensing section cannot be formed by merely incorporating transmitting and receiving antennas directly into a metal pipe section since metal will short out and/or distort the signal. Consequently, antennas are typically installed in non-conductive material. In the past, sensing sections have been formed by coating a section of metal pipe having a reduced diameter with an insulating material. The transmitter and receiver are placed on the insulating layer and then covered with a second insulating layer, such as fiberglass, rubber or epoxy, for protection. Since the body of the sensing section is composed of a metal pipe section, the sensing section can be readily connected into the bottom hole assembly using standard threaded metal couplings. A sensing section of this type is disclosed in the aforementioned U.S. Pat. No. 5,280,243 (Miller).
Unfortunately, this approach is not workable in small diameter drill strings. When the diameter of the metal pipe section supporting the insulating layers is reduced, the section becomes weaker and eventually is unable to withstand the mechanical forces imposed on the drill string. Moreover, in the case of MRI, the proximity of the metal pipe section interferes with the electromagnetic waves, thus distorting the analysis of the formation.
Consequently, it would be desirable to form a sensing section from a piping section formed from an electrically non-conductive and/or non-magnetic material, such as a composite material, so as to avoid the use of an underlying metallic pipe section. Unfortunately, this approach creates difficulties in joining the sensing section to the adjacent metallic members (e.g., the pipe couplings connecting the sensing section to the adjacent drill string sections). Traditional methods of joining non-metallic pipe sections, such as composite pipes, to metallic pipe couplings results in weak joints. Consequently, the large radial pressure gradient imposed across the joint as a result of the difference in the pressure of the drilling mud inside and outside of the drill string, combined with the high axial and torque loads, can cause failure of the joint (e.g., leaks).
Consequently, it would be desirable to provide an apparatus for connecting a section of an electrically non-conductive and/or non-magnetic conduit capable of carrying a pressurized fluid, such as a sensing section in a drill string, to a metallic coupling in such a way as to minimize the radial pressure gradient acting across the joint.